This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art.
Semiconducting polymers have been under extensive investigation due to their technological relevance in a wide range of applications from solar cells, light-emitting diodes, and transistors to various sensing platforms, among others. The capability of transporting charge carriers is one of the fundamental properties of semiconducting polymers. Efficient charge transport is strongly desired for such polymer-based thin-film devices, particularly for field-effect transistors and organic circuits. To date, a great number of conjugated polymers have been reported with charge carrier mobilities over 1 cm2 V−1 s−1 in both p-type and n-type transistors. A handful of donor-acceptor-type polymers have even shown hole mobility values exceeding 10 cm2 V−1 s−1. These inspiring and puzzling breakthroughs have far exceeded the charge transport limits for disordered polymers based on early theoretical models, mostly adapted from the study of inorganic semiconductors.
The discrepancy between experimental results and theoretical predictions has triggered a great deal of efforts to propose new theories to explain the efficient charge transport behaviors in polymer thin films. For examples, by studying an indacenodithiophene-benzothiadiazole copolymer, it has been argued in literature that charge transport in high-mobility semiconducting polymers is quasi one-dimensional, predominantly occurring along the backbone. This requires only occasional intermolecular hopping through short π-stacking bridges. Based on studies with the same polymer, researchers concluded that a planar, torsion-free polymer backbone with a low degree of energetic disorder is the origin for high charge carrier mobilities in donor-acceptor copolymers. Some researchers proposed a unified model of how charge carriers travel in conjugated polymer films from the study of a vast number of existing polymers. They argued that the limiting charge transport step is trapping caused by lattice disorder, and that short-range intermolecular aggregation is sufficient for efficient long-range charge transport. Hence, the unifying requirement for high carrier mobility is the presence of interconnected aggregates. These studies and others provide new insights into charge transport in polymeric thin films. However, a comprehensive understanding of transport in polymeric thin films is still lacking, and there is a need for enhancing these materials for the next generation of flexible and printed electronic device applications.
Conjugated polymers are promising optoelectronic materials for next-generation flexible and printed electronics. Conjugated polymers are a class of polymers with delocalized π-orbitals along the polymer backbone. Extensive efforts have been put into the design and synthesis of conjugated polymers. A plethora of knowledge about how to rationally control their optical, electronic, and redox properties has been realized in the development of numerous conjugated polymers. In contrast, partially-conjugated semiconducting polymers with intentionally-placed, non-conjugated flexible linkages along the polymer backbones have received little attention. There are two primary reasons. First, flexible linkages create high degrees of conformational and energetic disorder in polymer chains. Second, conjugation-break spacers (CBSs) disrupt the extended π-electron delocalization along polymer backbones. Conjugation-break spacers, also called non-conjugated spacers or non-conjugation spacers or non-conjugal spacers, are chemical groups that do not present pi-conjugation along the polymer backbone and connect two conjugated moieties through covalent bonds. In principle, both factors can have a negative influence on electronic properties, particularly for charge transport. On the other hand, high performance conjugated polymers are often plagued with poor solution-processability, a leading factor for batch-to-batch variations in both polymer synthesis and device fabrication. This limits applications of conjugated polymers in large scale flexible electronics. Conventionally, tuning solution-processability of polymers in organic solvents can be achieved by changing the size and shape of flexible solubilizing chains attached to polymer backbones. However, the modulation of polymer solution-processability and electronic performance turns out to be nontrivial. It should be noted that solution-processability is a term used to describe the desirable characteristics of the solution, namely adequate solubility of the polymer in this solvent, ability to make the solution as homogeneous as possible in terms the polymer concentration, and the ability of the solution to lend itself to depositing methods that provide uniform thickness and physical properties for films made from the solution.
Currently chlorinated solvents are utilized as solvents for polymer semiconductor materials for solution processing. These solvents are toxic. Further, the existing polymer semiconductors do not lend themselves to melt-processing. Further, the existing polymer semiconductors do not lend themselves extrusion and lamination processing.
For the forgoing reasons, there exists a need for approaches that can be applied to conjugated polymers to enhance their solution-processability as well as lending other types of processability for the conjugated polymers making them into useful polymer semiconductors for electronic and optoelectronic applications. Further, there is need for melt process able polymers and polymers that lens themselves to extrusion and lamination processing.